The invention relates to a single layer or multi-layer metal strip for the epitaxial coating with a biaxially textured layer and to a method for producing such metal strip. Such strip can be used advantageously as a backing strip for the deposition of biaxially textured layers of YBa2Cu3Ox superconducting material.
A metal strip, based on Ni, Cu and Ag, which is suitable for the epitaxial coating with a biaxially textured layer, is already known (U.S. Pat. Nos. 5,739,086, 5,741,377, 5,964,966 and 5,968,877). It is produced by cold rolling with a degree of deformation of more than 95% and subsequent recrystallization annealing, a sharp {001} <100 > texture (cubic texture) being formed.
Intensive work, particularly relating to the development of substrate materials based on Ni and Ag, is currently being carried out worldwide (J. E. Mathis et al., Jap. J. Appl. Phys. 37, 1998; T. A. Gladstone et al., Inst. Phys. Conf. Ser. No. 167, 1999).
One of the substrate materials developed consists of a nickel alloy, which is composed of Nia(Mob,Wc)dMe, in which M represents one or more metals other than Ni, Mo or W (DE 100 05 861 A1). In order to produce this material, initially an alloy of said composition is produced by melt metallurgy or powder metallurgy or by mechanical alloying and processed by thermoforming and by a subsequent, high quality, cold forming into a strip. This is subjected in a reducing or nonoxidizing atmosphere to a recrystallizing annealing. In comparison to technically pure nickel, the material has a higher grade, cubic texture, which is thermally more stable, and can be used as a substrate for physical, chemical coatings with a higher grade, microstructural orientation.
In the case of such materials, efforts are made to increase the strength. This is realized either by mixed crystal tempering, for which a nickel alloy with typically more than 5% of one or more alloying elements is rolled and recrystallized (U.S. Pat. No. 5,964,966; G. Celentano et al., Int. Journal of Modern Physics B, 13, 1999, page 1029, R. Nekkanti et al. Presentation at the Applied Supercond. Conf., Virginia Beach, Va., Sep. 17-27, 2000) or by rolling and recrystallizing a composite of nickel and a material of higher tensile strength (T. Watanabe et al., Presentation at the Applied Supercond. Conf., Virginia Beach, Va., Sep. 17-27, 2000).
For mixed crystal tempering, there is a critical degree of alloying, above which the cubic texture can no longer be formed. This phenomenon has been investigated intensively for brass alloys (Cu—Zn alloys with an increasing zinc content) and appears to be generally valid (H. Hu et al., Trans. AIME, 227, 1963, page 827, G. Wassermann, J. Grewen: Texturen metallischer Werkstoffe (Textures of Metallic Materials), Springer-Verlag Berlin/Göttingen/Heidelberg). Since the strength increases steadily with the alloy concentration, a maximum strength is also associated therewith. The second limitation is the high strength of the material already during the shaping by rolling. As a result, very large rolling forces arise already during the necessarily high degree of transformation so that, on the one hand, increased demands must be made on the rolling mill and, on the other, it becomes technically more difficult to carry out the exceptionally homogeneous deformation by rolling, which is required for forming the necessary, high-grade cubic texture.
When the strength of a composite of a Ni, Cu or Ag alloy with a material of higher strength is increased by rolling, there is also the problem of the high rolling forces when a material of great strength is deformed extensively. Because of the differences in the mechanical properties of the two materials, forming the composite, shear stresses occur at the interface during the rolling process and, with that, inhomogeneities in the deformation microstructure, which lower the cubic texture quality achievable during the recrystallization process.
One possibility of increasing the strength of a metallic matrix is the known dispersion tempering, for which preferably ceramic particles, finely dispersed in the matrix, are used. The particles can be introduced by powder metallurgical means or generated in situ by an exothermic reaction.
However, materials, produced in this manner, are not suitable for being processed by rolling and recrystallizing into a thin, biaxially textured strip. On the one hand, they already have a very high strength during the rolling and, on the other, it has not yet been possible to demonstrate the formation of strongly pronounced cubic texture, suitable for the application, in a dispersoid-containing strip.
Besides endeavors to increase the strength, efforts have been made to develop nonmagnetic substrates, in order to avoid hysteresis losses in alternating current applications (U.S. Pat. No. 5,964,966). Moreover, there have been attempts to use the substrate strip for stabilizing the current-carrying superconducting layer as a bypass (C. Cantoni et al., Presentation at the Applied Supercond. Conf., Virginia Beach, Va., Sep. 17-22, 2000). In order to realize this function, the substrate must have the highest possible electrical conductivity.